In the most widely used processes for removal of nitrogen oxides, nitrogen oxides are reduced to nitrogen with ammonia as the reducing agent. The reaction proceeds according to the equations indicated below: EQU 6NO+4NH.sub.3 .fwdarw.5N.sub.2 +6H.sub.2 O (1)
or, if O.sub.2 is present in the exhaust gas: EQU 4NO+4NH.sub.3 +O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O (2)